The present invention relates to a power transmission belt having a rib rubber layer which includes a plurality of ribs extending along a longitudinal direction of the belt on a belt base, and more particularly to improvement of endurance to rib chipping off.
As disclosed in the Japanese Unitily Model Registration Laying Open Gazette No. 54-60144, a power transmission belt in which short fibers such as natural fibers or synthetic fibers are oriented in a rubber of a transmission belt in the longitudinal direction of the belt in order to improve endurance to bending and endurance to cracking and consequently to improve durability of the belt has been known.
Also, the Japanese Patent Application Laying Open Gazette No. 62-219034 discloses sheets in which short fibers posses determined angle to the vertical direction of the belt by orienting short fibers in the longitudinal direction of the belt in an elastic sheet such as a rubber, etc., cutting this sheet at a determined angle, and adhering a plurality of this cut sheet.
In a V-ribbed belt having a plurality of ribs extending in parallel along the longitudinal direction of the belt under a belt base, a tension member layer for keeping a tension in order to transmit power is positioned above ribs to which the power from a pulley is applied directly. Therefore, large shearing stress is caused between a rib rubber and the tension member layer. Once a crack occurs, the crack grows rapidly and reaches to the tension member layer and rib chipping off is caused.
In order to solve the above problem, unit area load of a rib should be small. This results in increasing the belt size by 9 times from the limit size determined by the strength of the rib rubber and consequently increasing the cost.
On the other hand, as mentioned above, the strength-to-weight ratio of the rib rubber can be improved by mixing in the rib rubber the short fibers, which is oriented in a determined direction, and rib chipping off is restricted.
For example, the Japanese Patent Application Laying Open Gazette No. 62-188837 disclosed a V-ribbed belt mixed with short fibers oriented in the rib rubber layer in the widthwise direction of the belt in order to strengthen the rib rubber layer.
The direction of the shearing stress on the V-ribbed belt from the pulley is not uniform in a rib, but varies depending on parts of a rib. When the degree of orientation of the short fiber mixed in the rib rubber layer is preset at a uniform degree, that uniform degree may not suitable to the direction of the shearing stress in some parts. Thus, the strength-to-weight ratio was not fully improved.